When Grim has only 6 seconds left to live and delivers the catchphrase, "By the power of (the underworld/Greyskull) - I have the power!"; just like He-Man.
Wen Billy, Mandy & Grim turn into superheroes - their poses, the music and the silly words describing their actions (Slapp!!! Clobber!!! Goosh!!!) are all references to the Batman TV show.